47

Why the clamp
less core construction is preferred in the cold rolled oriented steel core?


Clamp less core construction
 The clamp type core construction
distorts the flux path and increases the eddy current therefore the clamp
less core construction is preferred in the cold rolled orientation steel
core.

48

How the
cooling of core is done in the transformer?


Cooling of core
 The cooling of core is done by
providing longitude oil ducts which are running parallel to the lamination or transverse ducts.

49

At which
condition, transverse ducts are provided in the transformer core?


Transverse ducts
 If the capacity of the transformer is
very high and circumscribing circle diameter greater than 0.8 meter, the heat
flows along the lamination.
 The transverse ducts improve the
cooling of the core in such a condition.

50

Define :
Window space factor


Window space factor
 It is defined as the ratio of copper
area to the total window area.

51

Write the
output equation of the transformer in terms of volts per turn.


Output equation of transformer
 Volts per turns = K√Q
 Where K = √ ( 4.44 × f × r × 10^{3 })
 Where r = Ð¤_{m}/AT

52

Describe the
condition for minimum cost, minimum losses, minimum weight, maximum
efficiency and minimum losses in the transformer.


Minimum cost : Cost of iron is equal to cost of
copper
Minimum losses : Variable loss is equal to constant
loss
Minimum weight : Weight of iron is equal to weight of
copper
Maximum efficiency : Variable loss is equal to constant
loss
Minimum losses : Iron loss is equal to copper loss

53

Describe the
effect of output and losses in the transformer if the linear dimension of two
transformer A and B are in the ratio of X : 1.


The output of the transformer A is X^{4}
times to that of transformer B whereas the losses in the transformer A is X^{3}
times to that of transformer B.

54

Why the value
of stacking factor always less than unity?


Stacking factor
 It is defined as the ratio of net
cross section area of core to the gross cross section area of the core.
 The value of stacking factor always
less than unity because the gross area of the core includes some space for
insulation coating and oil ducts.

55

Define :
Utilization factor


Utilization factor
 It is defined as the ratio of net
cross section area of core to the gross cross section area of the core.

56

What is meaning
of the specific iron loss?


It is iron loss per unit volume.

57

Describe the
importance of utilization factor.


Importance of utilization factor
 The utilization factor increases as
the number of step increases.
 Higher the utilization factor results
in increase in core area which will result in increase volt per turn for
specific flux density and core diameter.
 This will result in reduction of
winding copper material. Therefore the higher value of utilization factor better
for economy consideration.

58

Describe the
limitations of number of steps in the transformer.


The maximum number of steps for small
rating transformer is limited up to 6 and 15 for large rating transformer.

59

Which type of
core construction is preferred in the transformer?


Core type transformer : Rectangular, square or stepped
Shell type transformer : Rectangular

60

Why the
circular coils is preferred in the transformer?


Circular coil
 The circular coil has better
mechanical strength.
 The mechanical stress in the form of
circular due to excessive leakage in the case of short circuit.
 The shape of the circular coil does
not change due to radial forces during short circuit whereas the shape of
rectangular coil form a circular shape due to forces due to short circuit are
perpendicular to the conductors.

61

Why the
rectangular core is not used in the large power transformer?


Rectangular core
 The rectangular coil is not used in
the large power transformer because the diameter of circumscribing circle
increases which results in wastage of large space.
 This will result in length of mean
turn of winding increases which produces higher copper losses and cost of
conductor.

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